The present invention relates to a method for obtaining starch from maize (corn) involving soaking maize kernels in water to produce soaked maize kernels, grinding the soaked maize kernels to produce a ground maize slurry, and incubating the ground maize slurry with enzyme (e.g., protease).
To meet the expanding needs for ethanol and to be competitive with other petroleum based oxygenate additives, ethanol production costs must be lowered and the value of co-products increased. Approximately 60-70% of ethanol in the U.S. is produced by the conventional corn wet-milling process. The wet-milling process separates corn (maize) into a pure starch product and co-products rich in oil, fiber and protein. The corn is initially hydrated (steeped) in an aqueous solution of sulfur dioxide. The steeped corn is coarsely ground to loosen the intact germ from the kernel. Since germ contains a high concentration of oil (xcx9c45%), it is lighter than the other constituents of the ground slurry and can be separated (by density difference) by use of germ hydrocyclones. The remaining slurry is finely ground to disrupt the endosperm matrix and release the starch particles. Fiber particles are removed by passing the slurry over fine screens (75 xcexcm openings). Starch is separated from protein in a system of centrifuges and hydrocyclones, resulting in a starch fraction containing less than 0.35% (d.s.) protein. Starch is then further processed for different products such as ethanol or corn syrups.
The first and most important operation in the corn wet-milling process is steeping. Steeping involves soaking corn kernels counter-currently for 24-48 hours in warm (48xc2x0-54xc2x0 C.) sulfurous (0.1-0.2%) water. The purpose of steeping is to soften the corn kernel and to break the disulfide bonds holding the protein matrix together. Steeping is a diffusion limited process. The water and the steep chemicals (generally 2000-2500 ppm SO2 and 0.5-2% lactic acid (usually produced during steeping by lactobacillus bacteria)) diffuse into the corn kernel through the base end of the tip cap, move through the cross and tube cells of the pericarp to the kernel crown and into the endosperm. The SO2 in the endosperm reacts with the protein matrix that encapsulates the starch granules. The result is dispersion of endosperm protein and an enhancement of starch release during subsequent milling (Watson, S. A., et al., Cereal Chem., 38:22-23 (1961)). The penetration of SO2 into the endosperm and its reaction time with the protein matrix makes steeping a very time consuming operation (24 to 36 hours) in the corn wet-milling process. Steeping times shorter than 24 hours result in poor starch yields and loss of starch to fiber and protein fractions. Steeping is also one of the most capital and energy intensive parts of the corn wet-milling process. It is estimated that 21% of the total energy and capital cost is used for the steeping operation (Eckhoff, S. R., Wet milling short course, Course Notes, American Association of Cereal Chemists, St. Paul, Minn., 1999). Reducing steep time would decrease energy cost, increase plant capacity and reduce the capital cost involved in construction of new corn wet-milling plants.
Several mechanical and chemical approaches have been investigated to decrease steep time while maintaining product yields. These processes, however, required costly modifications of existing facilities or pretreatment of kernels, resulting in increased pollution or increased energy use (U.S. Pat. No. 3,597,274; Roushdi, M., et al., Starch/Stxc3xa4rke, 33: 7-9 (1981); Krochta, J. M., et al., J. Food Process. Preserv.,. 5: 39 (1981); Meuser, F., et al., 1985, The use of high-pressure disintegration technique for the extraction of starch from corn, pages 161-180, IN: New Approaches to Research on Cereal Carbohydrates, R. D. Hill and L. Munck, eds., Elsevier, Amsterdam; Hassanean, A., and A. Abdel-Wahed, Starch/Stxc3xa4rke, 38: 417 (1986); Grindel, R. S., Starch/Stxc3xa4rke, 17: 298 (1965); Neryng, A., and P. J. Reilly, Cereal Chem., 61: 8 (1984)).
The development of a processing procedure that could reduce the steep time and decrease or eliminate the use of chemicals such as sulfur dioxide would have a significant impact on the corn wet-milling industry. Such a process would appreciably decrease operational energy costs, increase plant capacity and reduce the capital costs involved in the construction of new corn wet-milling facilities.
A method for obtaining starch from maize involving soaking maize kernels in water to produce soaked maize kernels, grinding the soaked maize kernels to produce a ground maize slurry, and incubating the ground maize slurry with enzyme (e.g., protease).